This invention relates to liquid level sensing devices and more particularly to liquid level sensing devices applied to air conditioning and refrigeration systems. On refrigeration systems, this invention would be used for sensing oil level in a compressor crankcase as described in Barbier U.S. Pat. No. 5,103,648 or for detecting refrigerant liquid level in a receiver tank. In refrigeration systems, it is important that the all devices attached to the refrigerant piping be as pressure tight as possible because any emissions are considered as atmospheric pollutants. Therefore any service work required must be accomplished without requiring opening the piping system. For these reasons, it is important that level sensing devices be constructed so that the operating portion can be removed and serviced without opening the refrigerant containing piping system.
Present applications in receiver low and high level alarm applications use a mechanical float located inside of the receiver tank. As the float moves up and down with changes in liquid level, a rotational motion produced by the float motion moves a magnet. The motion of the magnetic field links through the tank wall with parts external of the pressurized system. The rotational motion of the float linked magnetically to an external moving arm is used to operate an electrical switching action. The Switchgauge made by Rochester Gauges is one of the types commonly used. This device provides a compact, low cost device which allows service of the electrical parts without opening the refrigeration system. Unfortunately, mechanical floats are subject to failure due to wear and dirt. For that reason, an opening in the tank wall must be provided for service removal for the float parts. Additionally the float arm length puts limits on its use in small tanks. Additionally the friction in the float and magnet mechanism reduces the accuracy of the switching point.
The present invention described herein overcomes these problems. By providing a compact level sensing device which uses a vertical optical window and associated electronic and electrical means for level detection, all moving parts inside of the pressurized vessel are eliminated. Accurate and repeatable level sensing is accomplished. Additionally the level sensing parts and associated power supply parts are designed to be compact and low in cost. To provide these functions in a small module, power for operating the light emitter must be accomplished from voltages as high as 240 v AC without the use of a transformer. Low voltage DC is required to supply electrical current to the light emitter and other electronic components. The heat produced in dropping the supply voltage from the supply line level to low voltage DC then becomes the major technical problem to solve. Light power is increased by any reduction in the optical distance by a function of the square of the ratio in the change in distance. Therefore optimum electronic construction for minimum light power requires that the light source and light detector be as close to the external prism face as possible to minimize the optical distance that light must travel to go from the light source to the internal prism reflective faces and back to the light detector. The present invention accomplishes this need by using a very efficient optical construction which reduces the heat loss in the power supply for module operation to an acceptable level. In addition the subject invention provides a construction of a level sensing device with a removable module for service that is self contained. In this way, the device is easily repaired by replacing a sealed water-tight module containing all the electronic, electrical and electro-optical components including the means to operate from various power supply voltages.